This invention relates to an improved expansive admixture for cement compositions and a method for cementing the annular space between the casing or pipe and surrounding formation in oil and gas wells.
In well bore cementing, the cement slurry is pumped down a pipe and up the annular space between the pipe and the surrounding earth formation. The slurry is allowed to set-up or harden in the annular space, thereby forming a rigid column which ideally forms a bond with the earth formation as well as with the metal pipe. Shrinkage in the cement column can result in small passageways, sometimes called microannuli, being formed across the diameter of the column. When this happens, formation fluids can migrate across the column and contaminate the producing zone. Inhibiting shrinkage in the cement column during setting serves to prevent the formation of microannuli and assures a good bond between the cement, pipe and formation.
Various methods have been suggested for inhibiting shrinkage of the cement column including the addition to the cement composition of gas delivering agents including powdered aluminum. See, for example, U.S. Pat. No. 2,120,468 to Noak Victor Hybinette entitled "Expanded Concrete and Process of Producing the Same", issued June 14, 1938.
In the co-pending application of Garry L. Briggs entitled "Resin Coated Aluminum" filed concurrently herewith, an aluminum expansive additive is disclosed which inhibits shrinkage in cement columns during well cementing operations. The novel expansive additive comprises aluminum particles which have been coated with a gasoline and water insoluble wood resin sold commercially as "VINSOL". The resin coated aluminum particles provide improved control over the nature of gas generation in the cement slurry and its duration and enable control of shrinkage in a cement column during its entire setting and hardening. The resin coating on the aluminum particles decreases the explosion hazard associated with handling aluminum particles while allowing the particles to be dry blended with a cement composition and stored in the shelf.
It has now been discovered that the novel resin coated aluminum particles in combination with activated charcoal and a fluid loss additive provide a greatly improved expansive admixture for cementing well bores.